Synergistic Thermal Conductivity Mechanism and Mechanical Properties of Graphene/Carbon Nanotubes Modified Cement for Geothermal Well

Summary

The efficient development of medium and deep geothermal energy, as well as the wellbore integrity of conventional production wells, is critically constrained by the low thermal conductivity and high cracking susceptibility of traditional cement sheaths. To address this issue, a high-thermal-conductivity modified cement was developed in this study by incorporating graphene and carbon nanotubes (CNTs) as synergistic nanofillers. The rheological properties, fluid loss under high-temperature and high-pressure (HTHP) conditions, mechanical performance, and microstructure of the modified cement were systematically evaluated. With the optimal formulation (0.10 wt% graphene + 0.10 wt% CNTs), the modified cement achieved a thermal conductivity of 1.46 W/(m·K) at 7 days, representing a 440% increase over that of plain cement (0.27 W/(m·K)). Analysis of the thermal conduction pathways revealed that the enhancement process exhibits a four-stage evolution, in which 1D CNTs act as thermal bridges connecting 2D graphene sheets. Meanwhile, its compressive strengths at 7 days and 28 days were maintained at 42.6 MPa and 55.5 MPa, respectively, significantly exceeding the requirements for complex well cementing. Thermogravimetric analysis (TGA) deconvolution and X-ray diffraction (XRD) semiquantitative peak area ratio analyses indicated that the synergistic nanofillers acted as nucleation sites, promoting the formation of stable hydration products such as tobermorite and xonotlite, thereby optimizing the phase composition. Scanning electron microscopy (SEM) observations further demonstrated that the 1D CNTs and 2D graphene constructed a dense cross-dimensional network for both thermal conduction and mechanical reinforcement through a “line-surface” interlaced architecture, effectively inhibiting microcrack propagation. Quantitative pore structure analysis by mercury intrusion porosimetry (MIP) confirmed that this network architecture optimized the pore size distribution and reduced the proportion of harmful pores. This study provides self-consistent experimental evidence for revealing the synergistic heat transfer behavior of multicomponent nanofillers in porous inorganic media. The thermal and mechanical performance data obtained in this study can lay a foundation for subsequent wellbore-scale numerical simulations and field pilot tests, while also indicating that graphene/CNTs modified cement possesses good application potential in complex downhole cementing engineering.